The present invention relates generally to integrated circuit designs, and more particularly to a system reducing or eliminating leakage between a pinned photodiode and shallow trench isolation fabricated therewith.
Semiconductor integrated circuit (IC) chips can be produced to function as image photosensors for image applications such as camera devices. A semiconductor photosensor produces signals that are proportional to light images by reading a predetermined number of individual pixels of an image. At least one device, such as a photodiode, is required for each pixel. By further incorporating a plurality of electronic reading and decoding circuitries, an array of image photosensors may digitally record an image with a predetermined pixel resolution.
Charge coupled devices (CCD) have been produced for this purpose with pinned photodiodes. In a pinned photodiode, a N+ region acts as a pixel sensor and is isolated from the surface of the IC by a P+ region. However, CCDs are typically difficult to integrate with logic circuits that might include metal-oxide-semiconductor field-effect-transistors (MOSFETs). Since MOSFETs are typically constructed with shallow trench isolation (STI), which is proved to be quite difficult to be integrated with pinned photodiodes, their mutual existence has been a challenging question for IC designers. In theory, STI is a device isolation structure of channels that are typically filled with oxide. The semiconductor crystal sidewalls of STI are damaged by the dry etch that is used to produce them. The sidewalls are further damaged by stresses that arise from the extreme temperature cycles that occur in semiconductor processing. The difficulty is that such damage causes electrical leakage if an electrical junction expands to such a damaged region.
Any electrical leakage appears as a signal in darkness. Such dark current reduces sensitivity and the quality of image depiction. Dark current can be generated by several typical structures in semiconductor devices. Electrical junctions that meet the oxide-covered surface are imperfect and can generate dark current. Shallow trench isolation between devices is produced by dry etching that damages the semiconductor crystal. If that damaged material is included in a junction depletion region, dark current can be generated.
Dark current is a bane of camera image quality. Therefore, desirable in the art of pinned photodiode designs are additional systems to integrate shallow trench isolation structures therewith, thereby improving image quality and overall IC performance. Prior attempts to reduce leakage and dark current typically do so at the expense of photodiode sensitivity. As such, it would be particularly desirable to provide a pinned photodiode that is resistant to leakage and dark current effects without compromising sensitivity.